Things You Will Need
A soft bristle brush, sponge, water hose, Depending on how badly the canvas is stained or if mildew stains are present, one or more of the following: Liquid Ivory soap or another mild neutral ph detergent, a solution of 50/50 water and isopropyl (rubbing) alcohol, 1/2 cup Lysol to a gallon of water or a cleaning product such as ISSO. If stains are persistent, a mixture of a non-chlorine bleach such as Clorox II in weak solution with water or as a last resort regular bleach (which can turn canvas yellow) or mildew products such as Tilex or X-14.
The first step to waterproofing cotton canvas is to clean it thoroughly. Brush off any loose dirt and hose the fabric with water. Mix one of the solutions mentioned above such as the Ivory soap with water (1/4 cup soap to 1 gallon water) and sponge into the stained area and let soak for several minutes. Scrub the canvas with the soft bristle brush while keeping the canvas wet with the soap solution. Then rinse thoroughly.
If stains remain after the initial cleaning, it will be necessary to use one of the other mixtures or products listed above. Begin by mixing the Clorox II and water and repeating Step Three above. If the stains are still present it may be necessary to use regular bleach, Tilex or X-14 to clean the area. I would do a small space to test the fabric for color fastness before cleaning the whole tarp or cover. If the color holds, repeat Step Three again.
The cleaning process will probably damage the waterproof finish of the fabric if there was any present before the cleaning process. After the fabric has completely dried, treat the fabric with a product such as Aqua-Tite, Camp-Dry or 303 High Tech Fabric Guard following the application instructions on the products container. This will restore the water repellency and extend the life of the fabric. The best way to apply a waterproofing product is to spray on one or two coats of the treatment allowing the first coat to dry completely before applying the second coat. It is best to apply the second coat perpendicular to the first. This will provide a more uniform coverage. Always test for colorfastness in a small inconspicuous area before treating the whole canvas.
You can help prevent future growth of mold and mildew by spraying on a mixture of 50% white vinegar and 50% water. A garden pump sprayer is good for this. Vinegar will not harm or stain the canvas.
Traditional cotton canvas gets its waterproof qualities from the cotton fibers swelling when they get wet sealing the weave. With the right cleaning tips and a little work a canvas tarp or cover can be cleaned to look new and then be treated with a waterproofing product to protect it.
Tips & Warnings
Start with the mildest soap solutions first to see if they will successfully remove the stains before using the stronger solutions. Wear gloves and safety goggles when working with the stronger solutions to protect your skin and eyes
Some waterproofing products give off noxious odors so the treatments should only be applied outdoors or in a well ventilated area.
Bleach and other chemicals used to clean mold and mildew can be poison and the vapors can be toxic to individuals with breathing problems. Some individuals also may have allergies that can be aggravated by the mold and mildew itself.